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    24V Control from 3.3V

    Hello,

    I am trying to drive a load of 24V and control that line by using a P MOSFET (U1)

    At gate of U1 I have a NMOS (2N7002) that control the gate signal

    The sch is given in attachment


    1. When I turn on Q1 but sending high to MECH_PWR_CNT, I get about 11V at Vg of U1

    The datasheet of U1 is also attached.

    The threshold voltage of U1 is -3V

    Vgs = 11-24V = -13V (Here MOSFET should turn on but its not, I rather see a spike and that again goes to zero), the same thing is true when I turn Q1 Off but at negative side.


    What am I doing wrong here

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    Re: 24V Control from 3.3V

    Hi,

    How do you measure it? Scope? DVM?
    Acidentally input in "AC mode"? --> use DC mode.

    What load do you drive? Current? Capacitive, inductive?

    What's the use of C3?

    Gate-to-GND is not of interest, please tell us gate-to-source voltage. Don't calculate it, but measure it directly at the Mosfet.

    Klaus
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    Re: 24V Control from 3.3V

    If Q1 is fully conducting you will get close to 0V at it's drain.
    R1 & R2 therefore produce a potential divider across 24V and 0V.
    So ~12V (really 12V + Q1 Vds) is what is expected. It works as designed.

    If you want to kill the 24_MECH completely, use a lower value for R2 and consider using an NPN BJT for U1. If it is being used as a switch it shouldn't need a large heat sink.

    Driving from 3.3V you are running perilously close to the Vgs threshold for a 2N7002, again, a bipolar transistor would be a better choice.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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    Re: 24V Control from 3.3V

    How do you measure it? Scope? DVM?
    SCOPE

    Acidentally input in "AC mode"? --> use DC mode.
    It is DC

    What load do you drive? Current? Capacitive, inductive?
    It will finally drive a motor and a thermal printer. Right now connected a resistor across.

    What's the use of C3?
    It is NC

    Gate-to-GND is not of interest, please tell us gate-to-source voltage. Don't calculate it, but measure it directly at the Mosfet.
    12V (ie Vs = 24V, Vg = 12V)

    - - - Updated - - -

    If Q1 is fully conducting you will get close to 0V at it's drain.
    Right

    R1 & R2 therefore produce a potential divider across 24V and 0V.
    Right

    So ~12V (really 12V + Q1 Vds) is what is expected. It works as designed.
    Right

    If you want to kill the 24_MECH completely, use a lower value for R2 and consider using an NPN BJT for U1. If it is being used as a switch it shouldn't need a large heat sink.
    What do you mean kill? Do you mean if I want drain voltage of U1 to go to zero when Q1 is turned off. If so then Yes, I do want that to happen.

    Driving from 3.3V you are running perilously close to the Vgs threshold for a 2N7002, again, a bipolar transistor would be a better choice.
    I will try a transistor.



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    Re: 24V Control from 3.3V

    Hi,

    Gate-to-GND is not of interest, please tell us gate-to-source voltage. Don't calculate it, but measure it directly at the Mosfet.
    12V (ie Vs = 24V, Vg = 12V)
    If you really measured 12V across the legs G-S of the MOSFET, then you donīt need to replace 2N7002.
    --> When V_GS is about 12V, then measure the voltage between Drain and Source of the power Mosfet..

    It will finally drive a motor and a thermal printer. Right now connected a resistor across.
    What about overcurrent and overvoltage protecton for the MOSFET?
    Why donīt you simply use a protected high side switch? Also called smart high side power switch, PROFET, and similar. Can be directly driven from 3V3 logic signal.

    Klaus
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    Re: 24V Control from 3.3V

    [QUOTE=KlausST;1634178]Hi,


    Its working now. The CRO Channel 1 wasn't working :(. I was trying this since yesterday


    There still a problem. The drain voltage never goes to zero when I turn off Q1. It holds on 1.6V

    What about overcurrent and overvoltage protecton for the MOSFET?
    Theres no protection on that

    Why donīt you simply use a protected high side switch? Also called smart high side power switch, PROFET, and similar. Can be directly driven from 3V3 logic signal.
    Good Option. Thx for suggesting. Do you know of any part numbers that can handle 24V @ about 5-6A and controlled by 3.3V



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    Re: 24V Control from 3.3V

    Hi,

    The drain voltage never goes to zero when I turn off Q1. It holds on 1.6V
    When the MOSFET is switched OFF, then this node is floating.
    Floating voltage is not defined it may be any voltage between 0V and 24V.
    Put a load (resistor) to get meaningful values. Or use a second transitor to switch the node to GND if you need this.

    Do you know of any part numbers that can handle 24V @ about 5-6A and controlled by 3.3V
    You want me to do the search for you?
    Every manufacturer as well as many distributors should have free selection tools / tables. Use them. They are free to use.

    Klaus
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    Re: 24V Control from 3.3V

    [When the MOSFET is switched OFF, then this node is floating. ]
    Vs = 24V and Vg = 24V and hence Vgs should be 0V and MOSFET Off. What node are you referring to.


    You want me to do the search for you?
    Not really :)
    No harm in asking if you knew ;)



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  9. #9
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    Re: 24V Control from 3.3V

    Hi,

    What node are you referring to.
    You talked about the drain voltage. --> Thus I referred to the drain voltage. This is the floating node.

    Btw: When you quote, please use the "Quote" button. (top tool bar, right side, icon = speech balloon)

    No harm in asking if you knew ;)
    True. But we are in different continents. With different price and availability, ... and you have to look for a device that fits your requirements.
    Thus I think it makes sense that every designer does the search on itīs own.
    It doesnīt take more time than posting all the requirements in a froum .. and you get best and fastest feedback.

    Klaus
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  10. #10
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    Re: 24V Control from 3.3V

    Btw: When you quote, please use the "Quote" button. (top tool bar, right side, icon = speech balloon)
    Sure.. Will do that.

    Thanks for your help. Appreciate it :)



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